In recent years, OTN (Optical Transport Network) has been standardized at ITU-T. OTN is based on the premise of wavelength-division multiplexing (WDM), with which the significant increase in Internet traffic can be accommodated. OTN has been standardized as a platform for performing transmission in a transparent manner. Specifically, in OTN transmission, an upper-level layer can totally disregard the lower-level layer, when transmitting client signals in end-to-end communication. Examples of client signals are those of a synchronous network such as Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH), as well as those of an asynchronous network such as Internet Protocol (IP) or Ethernet (registered trademark). The interface and the frame format of OTN have been standardized as the ITU-T standard G.709, and are rapidly being installed in commercial systems.
Consideration is given to multiplexing (multiplex storing) and demultiplexing a signal transfer frame having a low signal speed (Optical Channel Data Unit ‘j’:ODUj) and a signal transfer frame having a higher signal speed than ODUj (Optical Channel Data Unit ‘k’:ODUk), in a network where an interface complying with the ITU-T standard G.709 is applied.
For example, an ODU frame storing a client Ethernet (registered trademark) signal is referred to as a Lower Order ODU (LO_ODU), and an ODU frame storing plural low speed ODU frames by multiplexing is referred to as a Higher Order ODU (HO_ODU). That is to say, a low speed frame for transferring signals ODUj (for example, ODU1) is stored by multiplexing in a frame of a high signal speed HO_ODUk (including, for example, ODU2, ODU3, and ODU4). The low speed frame for transferring signals may be either LO_ODUj or HO_ODUj. That is to say, HO_ODUj may be multiplexed in HO_ODUk.
The operation of multiplexing ODUj frames in HO_ODUk is implemented by defining tributary slots (TS) that are time slots generated by dividing an OPUk (Optical Channel Payload Unit ‘k’) payload area, which is the payload part of a HO_ODUk frame, into ts areas in units of bytes, and storing ODUj in each TS of the payload area of the HO_ODUk frame.
The ITU-T standard G.709 defines two types of tributary slots (TS), i.e., one type having a bandwidth per TS of approximately 1.25 Gbps and another type having a bandwidth per TS of approximately 2.5 Gbps. When the bandwidth per TS is approximately 1.25 Gbps, the numbers of tributary slots ts are as illustrated in FIGS. 1A and 1B. Specifically, ts=2 for HO_ODU1, ts=8 for HO_ODU2, ts=32 for HO_ODU3, and ts=80 for HO_ODU4.
When the bandwidth per TS is approximately 2.5 Gbps, the numbers of tributary slots ts are as illustrated in FIG. 2. Specifically, ts=4 for HO_ODU2 and ts=16 for HO_ODU3. In FIGS. 1A through 2, TS#i (i=1 through 80) expresses tributary slots, OH expresses Overhead, FS expresses Fixed Stuff, and FEC expresses Forward Error Correction.
FIG. 3 illustrates how an ODU0 frame and an ODU1 frame are mapped into an OPU2 frame. In FIG. 3, an ODU0 frame is mapped into TS#1 of the payload area of the OPU2 frame, and an ODU1 frame is mapped into TS#4 and TS#8 of the payload area of the OPU2 frame. In this case, the number of tributary slots M occupied by the ODU1 in the payload area of HO_ODU2 is two.
The procedures of multiplexing ODUj into HO_ODUk are described below.
(1) According to the combination of ODUj, HO_ODUk, and the TS bandwidth, the multiplexing/demultiplexing method is determined to be either one of the following two methods. The first method is the Asynchronous Mapping Procedure (AMP) and the second method is the Generalized Mapping Procedure (GMP).
(2) According to the bandwidth (bit rate) of ODUj, the number of tributary slots M and the TS positions occupied by the ODUj in the payload area (OPUk) of HO_ODUk storing ODUj are determined.
(3) ODUj is stored in M number of tributary slots of HO_ODUk, while performing stuff processing by inserting null data according to the sum of the bandwidths of the M tributary slots and the difference of bandwidths of ODUj, by using the AMP method or the GMP method.
As described above, there are two methods, the AMP method and the GMP method, depending on the frequency adjustment method. The GMP method is a new method standardized when the ITU-T standard G.709 was revised in December, 2009. The AMP method is for performing multiplexing/demultiplexing while absorbing the frequency difference and the frequency deviation between the tributary slots (TS) of ODUj and HO_ODUk by inserting stuff in units of bytes (−1 through +2 bytes). The GMP method is for performing multiplexing/demultiplexing while absorbing the frequency difference and the frequency deviation between the tributary slots (TS) of ODUj and HO_ODUk by inserting stuff in units of M bytes. Here, M is the number of tributary slots (TS) of HO_ODU occupied when ODUj frames are stored into HO_ODU. Before the ITU-T standard G.709 was revised (before in December, 2009), multiplexing/demultiplexing was performed between signal frames by applying only the AMP method. However, presently, there is a need for performing multiplexing/demultiplexing between signal frames in an environment where both the AMP method and the GMP method are used. In this case, two separate processing units for the AMP method and the GMP method, and a circuit for selecting one of the two methods are needed. Thus, it is obvious that the scale of the system will be two times that of the case where only the prior method (AMP method) is used.
FIG. 4 illustrates a configuration of a conventional multiplexer/demultiplexer for multiplexing ODUj into HO_ODUk of OTUk, and demultiplexing ODUj from HO_ODUk of OTUk. The multiplexer/demultiplexer includes a demultiplexing block 10 and a multiplexing block 30. As an example, a description is given of procedures of demultiplexing ODUj from HO_ODUk of OTUk.
The demultiplexing block 10, which demultiplexes ODUj from HO_ODUk of OTUk, includes an OTUk processing unit 11, a HO_ODUk processing unit 12, a demultiplexer in units of bytes 13, an ODTU processing unit for AMP method 14, an ODTU processing unit for GMP method 15, a demapping unit for AMP method 16, a demapping unit for GMP method 17, selectors 18, 19, an ODUj processing unit 20, clock reproduction units 21, 22, and a clock generating unit 23.
The OTUk processing unit 11 extracts a HO_ODUk signal from an OTUk signal received from the network. At this time, the clock reproduction unit 21 extracts an OTUk clock from the received OTUk signal, and reproduces the OTUk clock. The clock generating unit 23 multiplies the OTUk clock by 239/255 and generates a HO_ODUk clock. The OTUk clock is generated in association with removing FEC from the OTUk signal and generating the HO_ODUk signal.
The demultiplexer 13 demultiplexes HO_ODUk signals in units of bytes and outputs them to the tributary slots TS#i.
The ODTU processing unit for AMP method 14 multiplexes, in units of bytes, the tributary slots TS#i in the OPUk payload area in which ODUj is stored, and forms an intermediate frame ODTUjk for the AMP method.
The ODTU processing unit for GMP method 15 multiplexes, in units of bytes, the tributary slots TS#i in the OPUk payload area in which ODUj is stored, and forms an intermediate frame ODTUk.ts for the GMP method.
The demapping unit for AMP method 16 extracts an ODUj signal from the intermediate frame ODTUjk, and supplies the ODUj signal from the selector 18 to the ODUj processing unit 20, and supplies the clock information of the intermediate frame ODTUjk from the selector 19 to the clock reproduction unit 22.
The demapping unit for GMP method 17 extracts an ODUj signal from the intermediate frame ODTUk.ts, and supplies the ODUj signal from the selector 18 to the ODUj processing unit 20, and supplies the clock information of the intermediate frame ODTUk.ts from the selector 19 to the clock reproduction unit 22.
The clock reproduction unit 22 uses the read enable information supplied from the demapping unit for AMP method 16 or the demapping unit for GMP method 17 and the HO_ODUk clock supplied from the clock generating unit 23, and regenerates an ODUj clock. The ODUj processing unit 20 outputs the ODUj supplied from the selector 18, with the use of the ODUj clock supplied from the clock reproduction unit 22.
Incidentally, there is proposed a demultiplexing converter including a first timeslot converting unit which is provided between a low speed interface and a high speed interface for converting the time slots in units of N×64 kb/s according to read control contents, a second timeslot converting unit which is provided between one or more high-speed interfaces for converting the time slots in units of tributary units TU-M according to read control contents, and a selecting unit for selecting between the first and second timeslot converting units (see, for example, patent document 1).
Furthermore, there is known a technology relevant to a shared circuit method in which a time division multiplex demultiplexing unit (MLDM) of a terminal apparatus of an optical transmission system is provided with a regenerating (REG) function. Specifically, the clocks used for reproducing/sending signals may be switched between incoming clocks to transmission line clock components and external clocks (see, for example, patent document 2).
Furthermore, there is proposed a demultiplexer including an inverse conversion processing circuit for demapping plural low-speed transmission signals from high-speed transmission signals, a switch processing circuit with which demultiplexed low-speed transmission signals and input low-speed transmission signals may be arbitrarily rearranged, and a multiplex processing circuit for mapping the plural low-speed transmission signals to high-speed transmission signals (see, for example, patent document 3).
Patent document 1: Japanese Laid-Open Patent Publication No. H3-208428
Patent document 2: Japanese Laid-Open Patent Publication No. H4-258043
Patent document 3: Japanese Laid-Open Patent Publication No. 2008-182540
With the demultiplexer illustrated in FIG. 4, in an environment where both the AMP method and the GMP method are used, in order to extract ODUj from HO_ODUk of OTUk, there is a need for ODTU processing units 14, 15 provided respectively for the AMP method and the GMP method, and demapping units 16, 17 provided respectively for the AMP method and the GMP method. In this case, the circuit scale would be two times as large as a circuit in which only the AMP method is used.